The voltage response of a display is typically non-linear. For conventional displays, the output luminance Y of a display may be related to an input value (e.g. an applied signal or control value such as input voltage V) by a power function, or gamma curve, as follows:Y∝Vγ  (1)where the gamma value γ (the numerical value of the exponent of the power function) is typically in the range of 1.8 to 3.5, and Y is the luminous intensity per unit area projected in a given direction, typically expressed in cd/m2 or nits. Conventionally, Y may be normalized to 1 relative to the luminance of a white reference which typically corresponds to a maximum luminance for the display (e.g. for a display having a white reference with a luminance of 200 cd/m2, Y=1 refers to a luminance value of 200 cd/m2). Similarly, input values may be normalized to 1 relative to a maximum input value. Normalized luminance values and normalized input values may be referred to as relative luminance values and relative input values, respectively.
The Rec. 709 standard of the International Telecommunication Union (ITU) uses a gamma value of 2.2. To help compensate for the expected voltage response of a display having a gamma value of 2.2, image data may be gamma-encoded or gamma-corrected with the inverse of the gamma value (i.e. encoded with a gamma value of about 1/2.2=0.45). FIG. 1 shows a gamma curve 8 (representing the voltage response of a display) having a gamma value of 2.2 and a gamma-encoding curve 9 having a gamma-encoding value of 1/2.2. As shown in the illustrated example of FIG. 1, if it is desired to display an image element (e.g. a pixel) with Y=0.218, then the original input value of V=0.218 is gamma corrected using gamma-encoding curve 9 to provide a gamma-corrected luminance value
  Y  =            V              1        γ              =                            (          0.218          )                          1          2.2                    =      0.5      as shown by arrow 6. When it is desired to display the image element, then the display is driven with the corresponding gamma-corrected input value V=0.5. Because of the non-linear display response curve 8, the input value V=0.5 provides the desired output luminance Y=Vγ=(0.5)2.2=0.218 as shown by arrow 7.
For conventional displays which typically have luminance levels of up to approximately 100 to 200 cd/m2, a single power law gamma curve (e.g. of the form of equation (1)) may be used to approximate the non-linear response of the display over its luminance range. At such luminance levels, the human visual system (HVS) perceives light in a non-linear fashion which, by coincidence, is approximately the inverse of the gamma curve of the display.
High brightness and/or high dynamic range (HDR) displays have evolved having a peak luminance as high as approximately 4000 cd/m2 or higher. At luminance levels beyond 200 cd/m2, and approaching 4000 cd/m2 or higher, the simple power law gamma-encoding curves become increasingly unsuitable for the HVS' perception of brightness, as the HVS perceives changes in brightness at higher luminance levels differently than at lower luminance levels.
High brightness and/or HDR displays may incorporate a spatially modulated light source such as those described in PCT Patent Application Publication Nos. WO02/069030, WO03/077013, WO2006/010244 and WO2008/092276. Such displays comprise a light source modulation layer (e.g. a spatially modulated backlight) and a display modulation layer. The light source modulation layer may be driven to produce a comparatively low-resolution representation of an image which is subsequently provided to the display modulation layer. The low-resolution representation is further modulated by the display modulation layer to provide a higher resolution image which is viewed by the observer. The light source modulation layer may comprise a matrix of actively modulated light sources, such as light emitting diodes (LEDs), for example. The display modulation layer, which may be positioned and/or aligned to receive light from the light source modulation layer, may comprise a liquid crystal display (LCD). The brightness of a pixel on the display modulation layer is therefore affected by the variable localized brightness across the light source modulation layer.
Because the light source modulation layer may produce a comparatively low-resolution representation of an image, the expected luminance pattern that will be provided on the display modulation layer when the driving values are applied to the light source modulation layer may be relatively slowly varying at the resolution of the display modulation layer. Therefore, it is possible to compute the expected luminance pattern at a lower resolution, and then to scale the expected luminance pattern up to a desired higher resolution (e.g. such as the resolution of the display modulation layer) without introducing significant artifacts.
The use of dual modulation layers having different resolutions may inhibit a simple one-to-one mapping between image data and output luminance values in a dual modulator display.
There is a general desire for systems and methods to process image data for high brightness and/or HDR displays.